Cellulose fibers or cellulose fiber containing materials are usually made hydrophobic (or "sized") with rosin. With internal sizing an emulsion of saponified rosin is added to the cellulose fibers dispersed in the stock together with alum for precipitation of the rosin on the fibers. This limits the choice of the pH of the pulp, since the precipitation of rosin must occur in an acid medium. These sizing systems are sensitive to high temperatures and consequently problems do often arise in modern paper making machines with closed white water systems.
Another type of sizing agent, the synthetic internal sizes, have recently begun to come into more extensive use. One advantage of these is that the pH-dependence during the sizing has been eliminated thereby making sizing possible in a neutral system. This improves the tensile properties of the finished paper. These synthetic internal sizes must be added in the form of water dispersions and a great problem with these is that up to the present there have been difficulties in producing dispersions with sufficiently good storage stability. The dispersions may for example coagulate or the sizing agent may be hydrolyzed.
The synthetic internal sizes consist of compounds containing a group, e.g. a ketene dimer, an anhydride, an acid chloride or a carbamoyl chloride group, which is capable of reacting with the hydroxyl groups of the cellulose fiber to form an irreversible bond.
Since the hydrophobic effect is obtained after a chemical reaction between the sizing agent and the cellulose fiber there is a great demand for proper dosage and for the maintaining of suitable reaction conditions.
Starch is used in many cases during paper making. By incorporation of starch in the pulp the mechanical strength of the paper is improved. The polar hydroxyl groups of the starch form cellulose fiber-starch-cellulose fiber bonds and thereby the dry strength of the paper is increased.